The operation of ground based radars, such as onboard automotive radars for detecting other vehicles in the same lane or an adjacent lane, can be impaired if the radar is blocked or misaligned in elevation angle. The radar can be blocked, for example, by layer of snow, slush, or mud on a radar dome (radome), or other secondary surface in the path of the radar signal. The radar beam can be misaligned in elevation, for example, during installation in the assembly plant, during replacement at dealerships, during operation due to vehicle loading, or following a collision. Blockage or vertical misalignment can impair the ability of the radar to detect objects to a point where operation of the system is compromised. In addition, it is sometimes a requirement that such radar systems are able to detect blockage or vertical misalignment so a vehicle operator can be informed that the radar system may not be operating properly.
Blockage detection techniques have been proposed that include: direct detection of the blockage layer using the same radar antenna elements used to detect objects about the ground vehicle, signal analysis techniques that analyze the amplitude or other waveform attributes of signals reflected by objects moving near the ground vehicle, and analyzing signals reflected by stationary objects. However, each of the above techniques has limited ability to reliably detect blockage. For example, the distance from the antenna to the blockage layer is typically less than the minimum range of the radar, and so is difficult to distinguish from low frequency leakage signals. Leakage signal cancellation has been found to be problematic due to unit to unit variation of radar system installations, and due to changes in ambient temperature. Furthermore, direct detection of a blockage layer may only indicate a signal reflection component caused by the blockage layer, and so may not provide an indication of signal absorption or loss caused by the blockage. As such, direct detection of the blockage layer may not be useful to determine total signal loss due to the blockage layer and so may not be useful to compensate or correct reflection signals from other objects.
Simple amplitude analysis of signals reflected by unknown objects is problematic due to the wide variation in reflectivity of unknown targets. Time averaging of reflected signals has been suggested, but it leads to undesirable signal latency. Current techniques of analyzing signals reflected by stationary objects include those using a fixed frequency waveform to enhance the signal spectrum, however, a limitation is that blockage detection then requires host vehicle motion, that is, blockage detection is not available when the host vehicle is stationary. It has been observed that the aforementioned techniques for blockage detection suffer from a high rate of missed or false blockage detections. Excessive latency can be an issue as well. Blockage techniques using a fixed frequency CW waveform cannot detect blockage when the host vehicle is stopped.
Furthermore the aforementioned techniques are not useful to detect vertical misalignment. Suggestions for detecting vertical misalignment include: using accelerometers to detect vertical motion and pitch angle changes, using vertical beam scanning to measure object elevation angle, and using radar image processing to analyze the range extent of the road edge. However, these suggestions undesirably increase cost.